1. Field of the Invention
The present invention relates to an ink-jet ink, and further particularly to an ink-jet ink which can provide images excellent in water fastness upon recording on non-coated paper (so-called plain paper) such as paper for copying, paper for reporting, notepaper, letter paper, bond paper and continuous business forms, which are commonly used in offices and homes, and is excellent in ejection stability and resistance to kogation and suitable for ink-jet recording systems using thermal energy.
2. Related Background Art
Inks with greatly various manners of being composed have heretofore been reported in respect of ink-jet inks. Such ink-jet inks are required to have the following performance characteristics:
(1) providing bright images high in optical density; PA1 (2) providing high-quality images free of any undefined or irregular feathering; PA1 (3) having high fixing ability to recording media; PA1 (4) providing prints excellent in water fastness and light fastness: PA1 (5) causing no clogging at minute ejection orifices of recording apparatus; PA1 (6) having good storage stability; PA1 (7) having high safety; and PA1 (8) being excellent in thermal ejection stability.
Of these performance characteristics, in recent years, the provision of images excellent in water fastness upon recording on so-called plain paper such as paper for copying, paper for reporting, notepaper, letter paper, bond paper and continuous business forms, which are commonly used in offices, is particularly required.
If an ink is used in an ink-jet recording apparatus of a system that droplets of the ink are ejected from an orifice by applying thermal energy from a heating element to the ink, thereby conducting recording, so-called kogation, i.e., a phenomenon that components such as a dye in the ink are thermally decomposed by this thermal energy from the heating element and built up on the heating element in the form of an insoluble matter, may be caused, whereby disadvantages such that a recorded image is deteriorated and the service life of the apparatus is shortened may be brought. Accordingly, it is important performance characteristics required of ink-jet inks to cause no kogation even when its ejection is repeated for a long period of time and not to adversely affect ejection property. It is also desirable that its ejection be more frequent.
When one attempts to prepare an ink satisfying other performance characteristics required of ink-jet inks as described above, there is however a problem that the composition of the ink comes to be a composition liable to cause kogation. For example, the provision of an image high in optical density requires to apply higher thermal energy to an ink, and a water-insoluble component must be contained as a composition satisfying the condition as to the water fastness of images, which has been required in recent years, so that the problem of kogation tends to occur. There is thus a demand for development of an ink-jet ink which provides images excellent in water fastness and moreover causes no kogation even when high thermal energy is applied to the ink.
Detailed researches and developments have been made from various approaches such as composition and physical properties of inks so that these conditions are satisfied. However, any ink satisfying all the conditions has not been yet found. For example, Japanese Patent Application Laid-Open Nos. 2-296878 and 2-255876 have proposed water-based ink compositions containing a polyamine. In such an ink, however, the hydrophilic group moiety of a dye is converted to a salt, so that the solution stability of the ink is lowered, and the lowering of reliability such as kogation and clogging at ejection orifices, and bronzing on a printed image are caused, resulting in uneven printing and/or insufficient optical density of the image. With a view toward enhancing the solution stability of an ink, it is also considered that a solution stabilizer is contained in the ink. In this instance, however, the solution stabilizer must be contained in a great amount, so that the quality of a recorded image formed by using such an ink is deteriorated.
Japanese Patent Application Laid-Open No. 3-91577 has proposed a method of improving the water fastness of the resulting image by converting a dye of a specific structure having a carboxyl group to an ammonium salt or volatile substituted ammonium salt and volatilizing ammonia or an amine out of the ink on a recording medium to form a free acid. Even in this instance, however, ammonia or the amine gradually volatilizes out of the ink, whereby the solution stability of the ink is lowered, which may often form the cause of kogation, clogging and lowering of stability of the ink, though the initial solution stability of the ink is improved.
With respect to the improvement of kogation, Japanese Patent Application Laid-Open No. 3-160070 mentions an improvement in preventing kogation by containing an oxo anion, in particular, a phosphate ion. In this instance, a divalent or trivalent metal ion may form an insoluble salt with the phosphate ion if such a metal ion exists in the ink or a flow path of the ink, so that the cause of kogation may be rather formed in some cases.